Question on identifiability in the “Dueling Network Architectures for Deep Reinforcement Learning” paper
Regarding your first question, $$V^{\pi}(s) = \sum_{a \in A}\pi(a|s)Q^{\pi}(s,a)$$ so recovering the value function from Q really depends on what policy $\pi$ you are using. Hence, you can't really recover the value function $V(s)$ from the $Q(s,a)$ values without knowing your policy distribution for state $s$.
However, you can recover $Q^{\pi}(s,a)$ values if we know $V^{\pi}(s)$ and $A^{\pi}(s,a)$. This is because
$$A^{\pi}(s,a) = V^{\pi}(s,a) - Q^{\pi}(s,a)$$
by definition of advantage. And this is why you need 2 heads to recover the $Q$ values from the Value and Advantage functions. In the original paper, the author's do not use this direct equation to recover $Q^{\pi}(s,a)$ values due to &quot;identifability&quot; issue and the fact that both $V^{\pi}(s)$ and $Q^{\pi}(s,a)$ are only estimates.
Regarding your second question, I believe the author's appllied the Duelling architecture on Double Deep Q Networks, which is an improvement over the single DQN used by Minh et al in learning atari. I do think that you can still use a a target network as in the single DQN case if you wanted to.